A conventional aromatic polycarbonate resin obtained by reacting 2,2-bis(4-hydroxyphenyl)propane (generally called “bisphenol A”) with phosgene or carbonic acid ester is excellent in thermal resistance and transparency and also in mechanical characteristics such as impact resistance and the like, and therefore is widely used as a structural material and also as an optical material for optical disc substrates, various types of lenses, prisms, optical fibers and the like.
Although having the thermal resistance and the mechanical strength, the conventional aromatic polycarbonate resin has a problem of a large photo-elastic coefficient when used as an optical film. For example, when the conventional aromatic polycarbonate resin is used as a phase film of a liquid crystal display and attached to a polarization plate, the following problem occurs. The phase film is stressed by thermal contraction, thermal expansion or the like of the polarization plate. As a result, phase spots appear and the uniformity of an image quality cannot be maintained.
The conventional aromatic polycarbonate resin also has the following problem when used as a phase plate. Even where a ¼ wave plate and a ½ wave plate are stacked together at an optimum angle in order to form a wide band circular polarization plate, the circular polarizability is insufficient over the entire wavelength range of visible light due to a large wavelength dependence of the phase contrast.
As a protective film for a polarization plate, triacetylcellulose (hereinafter, referred to as “TAC”) is mainly used especially in liquid crystal displays owing to superb characteristics thereof in terms of transparency, smoothness, thickness precision, adhesion and moisture permeability ratio. However, the liquid crystal displays are desired to have higher functions such as improved durability, polarization plate-protective films having a phase film function, and the like. In order to achieve such purposes, a polarization plate-protective film formed of cyclic polyolefin has been proposed. However, such a polarization plate-protective film has problems that air bubbles are generated and the film is deformed, for example, curled or warped, due to a low water vapor permeability thereof.
A known method for decreasing the photo-elastic coefficient of polycarbonate is described in, for example, Japanese Laid-Open Patent Publication No. 64-66234. According to this method, bisphenol A is copolymerized with tricyclo[5.2.1.02,6]decanedimethanol. However, this method decreases the thermal resistance and does not provide a sufficient effect of decreasing the photo-elastic coefficient. There is another known method of copolymerizing a bisphenol having a fluorene structure in a side chain with another bisphenol (see Japanese Laid-Open Patent Publications Nos. 6-25398 and 7-109342). With such a method, a bisphenol containing a fluorene structure is used at a high ratio in order to decrease the photo-elastic coefficient. As a result, the glass transition temperature is raised. Therefore, the process temperatures during the production of phase films, such as a temperature for film melting molding, a temperature for film stretching and the like are raised. This causes a problem of coloring, and also makes it difficult to produce the films with the film thickness, the phase contrast and the like being controlled with high precision.
According to a known method for controlling the wavelength dependence of a polycarbonate resin caused by the phase contrast, for example, a bisphenol having a fluorene structure in a side chain with another bisphenol (see the pamphlet of PCT International Publication WO2000/26705). However, as in the above-mentioned case of decreasing the photo-elastic coefficient, such a method uses a bisphenol having a fluorene structure at a high ratio and has a problem of a high glass transition temperature.
As a technology for improving the water vapor permeability of a polycarbonate resin, a method of forming microscopic holes in the film is known. However, such a film cannot be used as an optical film. As such, there has been no effective means for improving the water vapor permeability of a polycarbonate resin.